Ensure the master is stopped for good. For example, log in to the master
machine and use systemctlstoptarantool@<instance_name>.

Switch the replica to master mode by setting
box.cfg.read_only parameter to false and let
the load be handled by the replica (effective master).

Set up a replacement for the crashed master on a spare host, with
replication parameter set to replica
(effective master), so it begins to catch up with the new master’s state.
The new instance should have box.cfg.read_only
parameter set to true.

You lose the few transactions in the master
write ahead log file, which it may have not
transferred to the replica before crash. If you were able to salvage the master
.xlog file, you may be able to recover these. In order to do it:

Find out the position of the crashed master, as reflected on the new master.